The conventional dumbbell of the most primitive type is integrally made of a metal material by casting and is therefore provided with a fixed weight. The conventional dumbbell of the more advanced type is provided with a plurality of round weights joined by a short bar by which it is lifted or swung about in the hand for muscular exercise. Such a dumbbell is therefore adjustable in weight. The round weights are movable and can therefore fall on the ground accidentally to inflict a wound on the dumbbell user or bystanders.
In order to overcome the shortcomings of the conventional dumbbells described above, a prior art dumbbell is provided with a plurality of rectangular weights. The number of weights that can be lifted along with the dumbbell main body can be easily manipulated by changing the position of a fastening pin in a groove. In light of the fastening pin being located in the groove insecurely, the weights can accidentally fall on the ground at the time when the dumbbell is in use. After the prolonged use of the dumbbell, the fastening pin is vulnerable to deformation, thereby causing the fastening pin to unable to press securely against the connection rod of the weights.